In many wireless systems, communication nodes should be able to talk and to listen. This is needed in point-to-point communications connecting node A to node B, wherein the two nodes should be able to transmit information in both directions, namely from A to B and form B to A (see FIG. 1). One may be also interested to connect node A to node B (A talks and B listens) and node B to node C (B talks and C listens). It is desirable that such nodes can simultaneously talk and listen. An example is the telephone line which is composed of a single pair of wire, however, it can establish a two way connection and both parties involved in a conversation can simultaneously talk to each other and listen to each other. The operation of changing the direction of information flow in a communication link is called duplexing. A communication link with the capability to support the connection in both directions at the same time is called a full duplex, or a two-way system. In contrast, a link that can support the connection in only one direction at a time is called half duplex. It is important to realize that in many applications for point-to-point communication, the traffic is mostly in one direction (say from A to B) and the opposite link (from B to A) is used to communicate control information, for example ACK/NAK in an ARQ scheme or channel state information in adaptive transmission. This means it is important to establish the opposite link, even if it may have a small capacity. There are two methods commonly used for duplexing:
Time Division Duplex (TDD): This is indeed equivalent to half duplexing where the direction of the information flow can be changed in time to be in either from A to B or from B to A, but not in both directions at the same time. More generally, this corresponds to a system in which a given node can either talk or listen (can not talk and listen at the same time). TDD systems inherently suffer from high control overhead which is required to switch the connection from one direction to the other (this reduces the efficiency and increases the network complexity). TDD systems also suffer from an inherent delay in changing the direction of information flow which is problematic in some applications.
Frequency Division Duplex: This is equivalent to full duplexing, however, the two links use two different frequency bands. FDD systems are more complex due to using two units (transmitter and receiver units) operating in two different frequency bands. FDD systems are also less flexible in scenarios that the amount of traffic in one direction may be higher than the other (the allocated bandwidth determines the relative capacity of the two directions which is fixed depending on the hardware of the system and cannot be changed in response to changes in relative traffic loads in the two directions). To reduce hardware complexity, the bandwidth allocated to the two links are usually set equal to each other, which contradicts the fact that the traffic is usually higher in one direction in data centric applications (for example, in down-load of information in internet access the traffic is mostly from the access point to the mobile unit).
Disadvantages of the FDD,TDD Technologies: In summary, FDD systems inherently suffer from lack of flexibility and excessive hardware complexity, while TDD systems suffer from excessive control overhead and delay in switching the direction of information flow. It is desirable to have full-duplex systems with low hardware complexity and high flexibility.
Prior Relevant Works: It is widely known that one can improve the system capacity (as compared to TDD and FDD) by using a full duplex system wherein the two links overlap in both time and frequency [1][2]. The theory and the possibility of designing such a two-way radio system has been known for many years [3], however, bridging the gap between theory and practice in this area has been very challenging. There have been some prior works aiming to come up with a practical design, for example [4][5][6], however with limited success. One of the main differences between this invention and all these earlier works is that in our case the antennas are designed and/or activated (selected among the several available choices) to reduce the amount of the self interference: In one embodiment of this invention, the transmit antenna structure is designed to create static (designed in hardware and fixed) and/or dynamic (through beam-forming) null at the physical location of its corresponding receiver antenna. Alternatively, in another embodiment of this invention, the receive antenna structure is designed to create static (designed in hardware and fixed) and/or dynamic (though beam-forming) null at the physical location of its corresponding transmit antenna. In some embodiments of this invention, some specific designs to create such antenna structures based on patch antennas are given.